Ship bunkering calls for the best grades of bituminous coal, low in ash and preferably high in fixed carbon, because the use of low-grade coals would require carrying larger amounts, leaving less space for cargo. However, no country that has enough coal to bunker ships, need be dependent on foreign supplies; the low grade of coal would simply reduce efficiency and thus increase expense. =Substitutes.=--The proportion of coal used for power, as distinct from that used for heat and coal products, is increasing, and is now two-thirds of the total. As a source of power there is really no complete substitute for coal. All the great industrial nations, like England, Germany and the United States, have developed their industries on the basis of large coal supplies. Some countries make large use of hydro-electric power, but for most it is an insufficient substitute. Wood and other fuels are rarely sufficient to maintain an industry built up on a supply of coal. Oil is being successfully substituted in some industries, notably in shipping, but the importance of coaling stations will no doubt persist. CHANGES IN PRACTICE The technique of coal mining in many districts, and the development of heat, power and coal products are not far advanced. Wasteful methods are used, mostly as a result of competition and lack of co-operation and organization among producers. Economies are being advocated, however. Labor-saving machinery has been installed in many mines. A number of power plants have been erected near the mine mouth and the power distributed electrically, thus eliminating freight charges on coal. Central heating and power plants that can burn coal efficiently will no doubt be more popular and numerous in a few years. Government control and legislation may be expected to hasten the changes. In Europe the technique of coal mining, except in undercutting machinery, is further advanced than in the United States, as regards mining _all_ the coal and in supporting the surface. Improvements in coking ovens may soon make possible the manufacture of some sort of coke from almost any bituminous coal. While all coke may not be satisfactory for modern blast-furnace practice, any future lack of coke will probably be offset by the development of electric smelting, so the seriousness of the metallurgical need is doubtful. The proportion of by-product coke ovens, which make for cheaper coke by providing for other marketable products, is increasing. GEOLOGICAL DISTRIBUTION Coals are found in the sedimentary deposits of several geological eras: Paleozoic, Mesozoic, and Tertiary. The Paleozoic era, embracing the Carboniferous period, is by far the most important as regards quality and availability of its coal resources; but the lower-grade and chiefly lignitic coals of the Mesozoic and Tertiary are of great importance locally, and there are enormous reserves that exceed in quantity the generally higher-grade coals of the earlier periods.